Identification of object on interactive display surface by identifying coded pattern

ABSTRACT

A coded pattern applied to an object is identified when the object is placed on a display surface of an interactive display. The coded pattern is detected in an image of the display surface produced in response to reflected infrared (IR) light received from the coded pattern by an IR video camera disposed on an opposite side of the display surface from the object. The coded pattern can be either a circular, linear, matrix, variable bit length matrix, multi-level matrix, black/white (binary), or gray scale pattern. The coded pattern serves as an identifier of the object and includes a cue component and a code portion disposed in a predefined location relative to the cue component. A border region encompasses the cue component and the code portion and masks undesired noise that might interfere with decoding the code portion.

FIELD OF THE INVENTION

The present invention generally pertains to the use of encoded patternsto optically identify objects, and more specifically, pertains tovarious encoded patterns that can be applied to an object to enable theobject to be identified based on infrared light reflected from thepattern when the object is placed on or near the surface of aninteractive display surface.

BACKGROUND OF THE INVENTION

Visual codes such as UPC symbols are commonly used to associate a uniqueidentifier (a number or binary bit code) with a given physical object.This unique identifier is detected by employing image processingtechniques that analyze the arrangement of parts of the visual patternprinted on the object. In designing such visual coding schemes, it isimportant to consider whether the pattern may be easily detected byimaging the object, the ease with which the identifier may be recoveredfrom the pattern once the pattern is located in the image, and variousfeatures of the visual code such as the number of bits encoded, errordetection and/or correction, rotation dependence or invariance, etc.

An application in which a visual code must be decoded can often imposeconstraints on the form of the visual code used to provide an identifierfor an object. For example, the object on which the visual code isapplied may be relatively small, so that the visual code has to becompact. Furthermore, the pattern employed for visual coding must beperceptible to the imaging system used to read it within the resolutionconstraints of that system. If the small object has a particular shape,for example, a round “foot print” on the surface where the visual codeis to be detected, it would be desirable to use a circular visual codethat generally matches that round shape of the object. Similarly, if asmall object is rectangular or oval, it would be desirable for the shapeof the object to assist in determining the orientation of the visualcode, to facilitate decoding the pattern used in the code, to identifythe object.

Visual codes used for UPC bar codes on products sold in markets or otherretail establishments are typically read using a coherent light lasersource to scan the bar codes. Such laser scanning systems have arelatively high resolution and employ laser sources with sufficientlyhigh intensity to scan the bar codes without interference from ambientlight. In contrast, it is much more difficult to decode a visual code onan object within a image, particularly, when the image is of arelatively large surface, such as the display surface of an interactivedisplay. The imaging system must first detect an object in the image ofthe surface and then decode the pattern of the visual code applied tothe object.

Interactive displays that employ visual codes to identify objects areknown in the art. For example, an interactive display platform wasdeveloped in the MIT Media Lab, as reported by Brygg Ullmer and HiroshiIshii in “The metaDESK: Models and Prototypes for Tangible UserInterfaces,” Proceedings of UIST 10/1997:14–17. The metaDESK includes anear-horizontal graphical surface used to display two-dimensionalgeographical information. A computer vision system inside the desk unit(i.e., below the graphical surface) includes infrared (IR) lamps, an IRcamera, a video camera, a video projector, and mirrors. The mirrorsreflect the graphical image projected by the projector onto theunderside of the graphical display surface. The IR camera can detectreflected IR light from “hot mirrors,” comprising a material backing onthe undersurface of objects called “phicons” that are placed on thegraphical surface. In response to the IR camera detecting an IRreflection from this material, which is transparent to visible light,applied to the bottom of a “Great Dome phicon,” a map of the MIT campusis displayed on the graphical surface, with the actual location of theGreat Dome in the map positioned where the Great Dome phicon is located.The paper does not provide any teaching or suggestion of using an IRpattern and does not teach or explain details concerning the manner inwhich it is detected and identified, other than noting that for eachvision frame, the software produces “a list of unidentified ‘blobs’extracted from the scene.”

A similar approach to sensing objects on a display surface is disclosedin several papers published by Jun Rekimoto of Sony Computer ScienceLaboratory, Inc. in collaboration with others. These papers brieflydescribe a “HoloWall” and a “HoloTable,” both of which use IR light todetect objects that are proximate to or in contact with a displaysurface on which a rear-projected image is visible. The rear-projectionpanel, which is vertical in the HoloWall and horizontal in theHoloTable, is semi-opaque and diffusive, so that objects become moreclearly visible as they approach and then contact the panel. Althoughmention is made of using IR detectable patterns on objects foridentifying them, again, no details are provided about the IR pattern orabout how the IR pattern is detected to identify the objects.

Many types of visual codes are known in the prior art. However, giventhe limitations on resolution and other constraints mentioned above, itis likely that many of the prior art visual codes designed to work withhigh resolution scanners will not be suitable for use with a visionsystem that must use reflected IR light in an image of an entire displaysurface to read a visual code applied to a relatively small object inthe image. Accordingly, it would be desirable to employ visual codesthat are appropriate for the shape and size of the objects to beidentified, and which can be identified with a vision sensing systemhaving relatively limited resolution. A suitable encoded pattern shouldbe sufficiently easy to decode to enable realtime decoding while anobject having the encoded pattern is being moved about on a surface. Itwould be desirable that the pattern provide a relatively high contrastIR in the image of the surface, to enable the pattern to be decodedthrough a diffusing surface, since many prior art encoded patternsrequire too great a resolution to be usable on such a surface. A furtherdesirable characteristic of a suitable encoding pattern is that it havean innate orientation, to reduce the search space required to find theencoded pattern on a surface. For similar reasons, the encoded patternshould preferably be recognizable as a single connected component, tofacilitate detecting and decoding it. The prior art does not addressmany of these problems, which are more specific to a diffusing displaysurface and vision sensing system, as described above.

SUMMARY OF THE INVENTION

One of the advantages of the present invention is that it enables anappropriate identifier to be employed for an object that is used with aninteractive display table, based upon the shape and size of the object,the limitations imposed by the resolution of the vision sensing systemused to image the identifier and the requirements of the softwareapplication with which the object is used. One aspect of the inventionis thus directed to a two-dimensional identifier that is applied to anobject for encoding a value so that the value is determinable when thetwo-dimensional identifier is placed adjacent to the surface sensingsystem of the interactive table. The two-dimensional identifier includesa cue component having a contiguous area of a detectable material towhich the surface sensing system is responsive. The cue component isapproximated as an ellipse when detected by the surface sensing system,and the ellipse has axes that indicate an orientation of thetwo-dimensional identifier relative to a coordinate system of thesurface sensing system. A code portion of the identifier is disposed ina predefined location relative to the cue component. The code portionencodes the value with at least one binary element that is detectable bythe surface sensing system. Also included is a border region thatencompasses the cue component and the code portion. The border regionincludes a non-detectable material that is not sensed as part of thetwo-dimensional identifier by the surface sensing system and whichfunctions as an interference mask around the cue component and the codeportion, to minimize noise. The detectable material comprises areflective material that reflects infrared light to which the surfacesensing system responds.

Several different types of encoding are used in the identifier. In one,a radial code is used for encoding the value, and in this type ofencoding, the cue component comprises a contiguous radial area of thedetectable material. The radial area includes a sub-area of thenon-detectable material disposed at a first predefined radius from acenter of the radial area. The sub-area represents a start bit thatindicates a start location from which the code portion is to be read.Also, the code portion is configured in a region disposed at a secondpredefined radius from the center of the radial area, and the secondpredefined radius is greater than the first predefined radius, so thatthe code portion generally extends around the cue component.

Another encoding scheme employs a variable length linear code forencoding the value. In this scheme, the cue component comprises parallelstrips of the detectable material that are of substantially equal lengthand are connected at a first end by a predefined area of the detectablematerial used as a start bit. The parallel strips are separated by thenon-detectable material at a second end, which is opposite the firstend, and the non-detectable material at the second end is used as an endbit. The code portion includes a pattern of detectable areas andundetectable areas. Each detectable area and each undetectable area isof predefined dimensions and is disposed between the parallel strips andbetween the start bit and the end bit.

In another encoding scheme, a variable bit-length code is used forencoding the value. In this case, the cue component includes apredefined dimension and a variable length dimension. The predefineddimension is always smaller than the variable length dimension, and thevariable length dimension indicates a reference orientation and anextent of the code portion. The code portion begins with a first binaryelement at a predefined offset from the cue component, and anyadditional binary elements extend away from the first binary element inat least one of three ways, including in a direction generally parallelto the reference orientation of the cue component, to form a row ofbinary elements that ends at the extent of the code portion indicated bythe variable length dimension; or in a direction generally perpendicularto the variable length dimension, to form a matrix of binary elementsthat end when a row does not include at least one binary element that isdetectable by the surface sensing system; or in predefined directionsand predefined distances relative to an immediately preceding binaryelement, to form a series of binary elements at predefined locationsrelative to each other.

In another type of encoding, the value is encoded using a multi-level,variable bit-length code, so that the cue component is detectable at abinarization threshold and has a predefined shape with at least onevariable length dimension. The at least one variable length dimensionindicates a reference orientation and an extent of the code portion. Thecode portion comprises at least one binary element that is detectable ata gray scale threshold, but is not detectable at the binarizationthreshold. The at least one binary element that is detectable at thegray scale threshold is disposed within the cue component so that thecue component is uniform at the binarization threshold, but reveals theat least one binary element at the gray scale threshold. Also, the codeportion can begin with a first binary element disposed at a predefinedoffset from a local origin and disposed at a predefined locationrelative to the cue component; any additional binary elements extendaway from the first binary element in at least one of a directiongenerally parallel to an axis used by the surface sensing system, andpredefined directions and predefined distances relative to animmediately preceding binary element, to form a series of binaryelements at predefined locations relative to each other so that the codeportion comprises a predefined shape.

Another aspect of the present invention is directed to a method fordetermining a value from a two-dimensional identifier applied to anobject when the object is placed adjacent to a surface of a surfacesensing system. The method detects identifiers generally like thosediscussed above.

Yet another aspect of the present invention is directed to a system fordetermining a value from a two-dimensional identifier applied to anobject that includes an interactive display surface having aninteractive side adjacent to which the object can be placed, and anopposite side. A light source in the system directs IR light toward theopposite side of the interactive display surface and through theinteractive display surface, to the interactive side. A light sensor isdisposed to receive and sense IR light reflected back from the patternedobject through the interactive display surface, forming an image thatincludes the two-dimensional identifier applied to the object. Aprocessor that is in communication with the light sensor executesmachine instructions stored in a memory causing the processor to carryout a plurality of functions that are generally consistent with thesteps of the method used for determining the value encoded by anidentifier applied to an object placed on or near the interactivedisplay surface.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of a generally conventionalcomputing device or personal computer (PC) that is suitable for imageprocessing for the interactive display table as used in practicing thepresent invention;

FIG. 2 is an illustration of the interior of the interactive displaytable showing hardware components included, and the paths followed bylight within the interactive display table, and exemplary objectsdisposed on and above the surface of the interactive display table;

FIG. 3 is an isometric view of an interactive display table coupled tothe PC externally;

FIG. 4 is a flow chart illustrating the overall logic for recognizing atwo-dimensional identifier code that is applied to an object placed onor near the display surface;

FIG. 5 illustrates an exemplary radial code in accord with the presentinvention;

FIG. 6 is a flow chart illustrating the logic steps performed by animage processing module when determining a radial code applied to anobject on the display surface;

FIG. 7 illustrates an exemplary matrix code in accord with the presentinvention;

FIG. 8 is a flow chart showing the logical steps logic steps performedby an image processing module for determining a die matrix code;

FIG. 9 illustrates an exemplary linear code in accord with the presentinvention;

FIG. 10 is a flow chart illustrating the logical steps logic stepsperformed by the image processing module for determining a linear code;

FIG. 11 illustrates an exemplary variable bit-length matrix code inaccord with the present invention;

FIG. 12 is a flow chart showing the logical steps for determining avariable bit-length matrix code applied to an object;

FIG. 13 illustrates exemplary gray scale codes in accord with thepresent invention;

FIG. 14 illustrates an exemplary arbitrary shape code in accord with thepresent invention;

FIG. 14A illustrates an exemplary object with the arbitrary shape codeof FIG. 14, and having a user activated button for modifying the shapecode; and

FIG. 15 is a flow chart illustrating the logical steps performed by theimage processing module for determining a gray scale or arbitrary shapecode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary Computing System for Implementing Present Invention

With reference to FIG. 1, an exemplary system suitable for implementingvarious portions of the present invention is shown. The system includesa general purpose computing device in the form of a conventional PC 20,provided with a processing unit 21, a system memory 22, and a system bus23. The system bus couples various system components including thesystem memory to processing unit 21 and may be any of several types ofbus structures, including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. The system memory includes read only memory (ROM) 24 andrandom access memory (RAM) 25. A basic input/output system 26 (BIOS),containing the basic routines that help to transfer information betweenelements within the PC 20, such as during start up, is stored in ROM 24.PC 20 further includes a hard disk drive 27 for reading from and writingto a hard disk (not shown), a magnetic disk drive 28 for reading from orwriting to a removable magnetic disk 29, and an optical disk drive 30for reading from or writing to a removable optical disk 31, such as acompact disk-read only memory (CD-ROM) or other optical media. Hard diskdrive 27, magnetic disk drive 28, and optical disk drive 30 areconnected to system bus 23 by a hard disk drive interface 32, a magneticdisk drive interface 33, and an optical disk drive interface 34,respectively. The drives and their associated computer readable mediaprovide nonvolatile storage of computer readable machine instructions,data structures, program modules, and other data for PC 20. Although theexemplary environment described herein employs a hard disk, removablemagnetic disk 29, and removable optical disk 31, it will be appreciatedby those skilled in the art that other types of computer readable media,which can store data and machine instructions that are accessible by acomputer, such as magnetic cassettes, flash memory cards, digital videodisks (DVDs), Bernoulli cartridges, RAMs, ROMs, and the like, may alsobe used in the exemplary operating environment.

A number of program modules may be stored on the hard disk, magneticdisk 29, optical disk 31, ROM 24, or RAM 25, including an operatingsystem 35, one or more application programs 36, other program modules37, and program data 38. A user may enter commands and information in PC20 and provide control input through input devices, such as a keyboard40 and a pointing device 42. Pointing device 42 may include a mouse,stylus, wireless remote control, or other pointer, but in connectionwith the present invention, such conventional pointing devices may beomitted, since the user can employ the interactive display for input andcontrol. As used hereinafter, the term “mouse” is intended to encompassvirtually any pointing device that is useful for controlling theposition of a cursor on the screen. Other input devices (not shown) mayinclude a microphone, joystick, haptic joystick, yoke, foot pedals, gamepad, satellite dish, scanner, or the like. These and other input/output(I/O) devices are often connected to processing unit 21 through an I/Ointerface 46 that is coupled to the system bus 23. The term I/Ointerface is intended to encompass each interface specifically used fora serial port, a parallel port, a game port, a keyboard port, and/or auniversal serial bus (USB). System bus 23 is also connected to a camerainterface 59, which is coupled to an interactive display 60 to receivesignals form a digital video camera that is included therein, asdiscussed below. The digital video camera may be instead coupled to anappropriate serial I/O port, such as to a USB version 2.0 port.Optionally, a monitor 47 can be connected to system bus 23 via anappropriate interface, such as a video adapter 48; however, theinteractive display table of the present invention can provide a muchricher display and interact with the user for input of information andcontrol of software applications and is therefore preferably coupled tothe video adaptor. It will be appreciated that PCs are often coupled toother peripheral output devices (not shown), such as speakers (through asound card or other audio interface—not shown) and printers.

The present invention may be practiced on a single machine, although PC20 can also operate in a networked environment using logical connectionsto one or more remote computers, such as a remote computer 49. Remotecomputer 49 may be another PC, a server (which is typically generallyconfigured much like PC 20), a router, a network PC, a peer device, or asatellite or other common network node, and typically includes many orall of the elements described above in connection with PC 20, althoughonly an external memory storage device 50 has been illustrated inFIG. 1. The logical connections depicted in FIG. 1 include a local areanetwork (LAN) 51 and a wide area network (WAN) 52. Such networkingenvironments are common in offices, enterprise wide computer networks,intranets, and the Internet.

When used in a LAN networking environment, PC 20 is connected to LAN 51through a network interface or adapter 53. When used in a WAN networkingenvironment, PC 20 typically includes a modem 54, or other means such asa cable modem, Digital Subscriber Line (DSL) interface, or an IntegratedService Digital Network (ISDN) interface for establishing communicationsover WAN 52, such as the Internet. Modem 54, which may be internal orexternal, is connected to the system bus 23 or coupled to the bus viaI/O device interface 46, i.e., through a serial port. In a networkedenvironment, program modules, or portions thereof, used by PC 20 may bestored in the remote memory storage device. It will be appreciated thatthe network connections shown are exemplary and other means ofestablishing a communications link between the computers may be used,such as wireless communication and wide band network links.

Exemplary Interactive Surface

In FIG. 2, an exemplary interactive display table 60 is shown thatincludes PC 20 within a frame 62 and which serves as both an opticalinput and video display device for the computer. In this cut-away Figureof the interactive display table, rays of light used for displaying textand graphic images are generally illustrated using dotted lines, whilerays of infrared (IR) light used for sensing objects on or just above adisplay surface 64 a of the interactive display table are illustratedusing dash lines. Display surface 64 a is set within an upper surface 64of the interactive display table. The perimeter of the table surface isuseful for supporting a user's arms or other objects, including objectsthat may be used to interact with the graphic images or virtualenvironment being displayed on display surface 64 a.

IR light sources 66 preferably comprise a plurality of IR light emittingdiodes (LEDs) and are mounted on the interior side of frame 62. The IRlight that is produced by IR light sources 66 is directed upwardlytoward the underside of display surface 64 a, as indicated by dash lines78 a, 78 b, and 78 c. The IR light from IR light sources 66 is reflectedfrom any objects that are atop or proximate to the display surface afterpassing through a translucent layer 64 b of the table, comprising asheet of vellum or other suitable translucent material with lightdiffusing properties. Although only one IR source 66 is shown, it willbe appreciated that a plurality of such IR sources may be mounted atspaced-apart locations around the interior sides of frame 62 to prove aneven illumination of display surface 64 a. The infrared light producedby the IR sources may:

-   -   exit through the table surface without illuminating any objects,        as indicated by dash line 78 a;    -   illuminate objects on the table surface, as indicated by dash        line 78 b; or    -   illuminate objects a short distance above the table surface but        not touching the table surface, as indicated by dash line 78 c.

Objects above display surface 64 a include a “touch” object 76 a thatrests atop the display surface and a “hover” object 76 b that is closeto but not in actual contact with the display surface. As a result ofusing translucent layer 64 b under the display surface to diffuse the IRlight passing through the display surface, as an object approaches thetop of display surface 64 a, the amount of IR light that is reflected bythe object increases to a maximum level that is achieved when the objectis actually in contact with the display surface.

A digital video camera 68 is mounted to frame 62 below display surface64 a in a position appropriate to receive IR light that is reflectedfrom any touch object or hover object disposed above display surface 64a. Digital video camera 68 is equipped with an IR pass filter 86 a thattransmits only IR light and blocks ambient visible light travelingthrough display surface 64 a along dotted line 84 a. A baffle 79 isdisposed between IR source 66 and the digital video camera to prevent IRlight that is directly emitted from the IR source from entering thedigital video camera, since it is preferable that this digital videocamera should produce an output signal that is only responsive to the IRlight reflected from objects that are a short distance above or incontact with display surface 64 a and corresponds to an image of IRlight reflected from objects on or above the display surface. It will beapparent that digital video camera 68 will also respond to any IR lightincluded in the ambient light that passes through display surface 64 afrom above and into the interior of the interactive display (e.g.,ambient IR light that also travels along the path indicated by dottedline 84 a).

IR light reflected from objects on or above the table surface may be:

-   -   reflected back through translucent layer 64 b, through IR pass        filter 86 a and into the lens of digital video camera 68, as        indicated by dash lines 80 a and 80 b; or    -   reflected or absorbed by other interior surfaces within the        interactive display without entering the lens of digital video        camera 68, as indicated by dash line 80 c.

Translucent layer 64 b diffuses both incident and reflected IR light.Thus, as explained above, “hover” objects that are closer to displaysurface 64 a will reflect more IR light back to digital video camera 68than objects of the same reflectivity that are farther away from thedisplay surface. Digital video camera 68 senses the IR light reflectedfrom “touch” and “hover” objects within its imaging field and produces adigital signal corresponding to images of the reflected IR light that isinput to PC 20 for processing to determine a location of each suchobject, and optionally, the size, orientation, and shape of the object.It should be noted that a portion of an object (such as a user'sforearm) may be above the table while another portion (such as theuser's finger) is in contact with the display surface. In addition, anobject may include an IR light reflective pattern or coded identifier(e.g., a bar code) on its bottom surface that is specific to that objector to a class of related objects of which that object is a member.Accordingly, the imaging signal from digital video camera 68 can also beused for detecting each such specific object, as well as determining itsorientation, based on the IR light reflected from its reflectivepattern, in accord with the present invention. The logical stepsimplemented to carry out this function are explained below.

PC 20 may be integral to interactive display table 60 as shown in FIG.2, or alternatively, may instead be external to the interactive displaytable, as shown in the embodiment of FIG. 3. In FIG. 3, an interactivedisplay table 60′ is connected through a data cable 63 to an external PC20 (which includes optional monitor 47, as mentioned above). As alsoshown in this Figure, a set of orthogonal X and Y axes are associatedwith display surface 64 a, as well as an origin indicated by “0.” Whilenot discretely shown, it will be appreciated that a plurality ofcoordinate locations along each orthogonal axis can be employed tospecify any location on display surface 64 a.

If the interactive display table is connected to an external PC 20 (asin FIG. 3) or to some other type of external computing device, such as aset top box, video game, laptop computer, or media computer (not shown),then the interactive display table comprises an input/output device.Power for the interactive display table is provided through a power lead61, which is coupled to a conventional alternating current (AC) source(not shown). Data cable 63, which connects to interactive display table60′, can be coupled to a USB 2.0 port, an Institute of Electrical andElectronics Engineers (IEEE) 1394 (or Firewire) port, or an Ethernetport on PC 20. It is also contemplated that as the speed of wirelessconnections continues to improve, the interactive display table mightalso be connected to a computing device such as PC 20 via such a highspeed wireless connection, or via some other appropriate wired orwireless data communication link. Whether included internally as anintegral part of the interactive display, or externally, PC 20 executesalgorithms for processing the digital images from digital video camera68 and executes software applications that are designed to use the moreintuitive user interface functionality of interactive display table 60to good advantage, as well as executing other software applications thatare not specifically designed to make use of such functionality, but canstill make good use of the input and output capability of theinteractive display table.

An important and powerful feature of the interactive display table(i.e., of either embodiments discussed above) is its ability to displaygraphic images or a virtual environment for games or other softwareapplications and to enable an interaction between the graphic image orvirtual environment visible on display surface 64 a and identify objectsthat are resting atop the display surface, such as an object 76 a, orare hovering just above it, such as an object 76 b.

Again referring to FIG. 2, interactive display table 60 includes a videoprojector 70 that is used to display graphic images, a virtualenvironment, or text information on display surface 64 a. The videoprojector is preferably of a liquid crystal display (LCD) or digitallight processor (DLP) type, with a resolution of at least 640×480pixels. An IR cut filter 86 b is mounted in front of the projector lensof video projector 70 to prevent IR light emitted by the video projectorfrom entering the interior of the interactive display table where the IRlight might interfere with the IR light reflected from object(s) on orabove display surface 64 a. A first mirror assembly 72 a directsprojected light traveling from the projector lens along dotted path 82 athrough a transparent opening 90 a in frame 62, so that the projectedlight is incident on a second mirror assembly 72 b. Second mirrorassembly 72 b reflects the projected light onto translucent layer 64 b,which is at the focal point of the projector lens, so that the projectedimage is visible and in focus on display surface 64 a for viewing.

Alignment devices 74 a and 74 b are provided and include threaded rodsand rotatable adjustment nuts 74 c for adjusting the angles of the firstand second mirror assemblies to ensure that the image projected onto thedisplay surface is aligned with the display surface. In addition todirecting the projected image in a desired direction, the use of thesetwo mirror assemblies provides a longer path between projector 70 andtranslucent layer 64 b to enable a longer focal length (and lower cost)projector lens to be used with the projector.

Object Code Recognition

In FIG. 4 a flow diagram illustrates overall logic for recognizing atwo-dimensional identifier code of a coded object. In a step 100, anapplication program, such as a game, specifies the type(s) of code(s)that the application expects to find attached to objects on the table.Code types usable in the present invention include circular, linear,matrix, variable bit length matrix, multi-level matrix, black/white(binary), and gray scale. Specifying the code type(s) can includeloading code pattern characteristics, setting threshold values, and/orenabling templates for pattern recognition. Enabling a template can alsoinclude generating several rotated versions of the template tofacilitate object recognition.

The remaining logic of FIG. 4 is preferably performed by an imageprocessing module that communicates with a software applicationcurrently being executed. In this way, any software application caninterface with the image processing module in a standardized manner. Asdescribed in further detail below, the image processing module performsa variety of functions related to detecting objects through shapes,pixel intensities, code types, and corresponding code values. Examplesinclude template matching or thresholding to attempt to find a codedobject wherever the object may appear on the table and at anyorientation.

In a step 102, the image processing module detects a connected componentin a binary image of a current input image produced by the IR videocamera in response to reflected IR light received from the displaysurface of the table, from one or more objects that are on or near thedisplay surface. In this exemplary embodiment of an IR vision system,the connected component comprises a set of adjacent “on” pixels in thebinary image. The pixels include those portions of a two-dimensionalidentifier code comprising an IR reflective material that have reflectedIR light back toward the IR video camera. The connected component cancorrespond to the entire identifier code attached to an object placed onthe interactive table, or to a sub-component that is separate from acode portion of the identifier code. In some instances described below,the separate sub-component acts as a cue-component to help determine theposition and orientation of the identifier code. In other instances, thewhole identifier code serves as its own cue component for locating theposition and orientation of the identifier code. The binary image isformed by determining which pixels of a normalized input image are abovea predetermined binarization threshold. The connected componentdetection is performed using standard algorithms such as algorithmsdescribed by B. K. P. Horn (see Robot Vision).

In a step 104, the image processing module represents the connectedcomponent as an ellipse, and calculates a center and lengths of themajor and minor axes of the ellipse. The image processing module thendetermines the mean and covariance values for pixel of the connectedcomponent to determine the position and orientation of the connectedcomponent relative to the interactive display table coordinates, basedupon the statistical mean and covariance of pixels along the ellipticalaxes. For details of this technique, see “Connected Components Analysis”by Andrew Hunter, available atwww.dur.ac.uk/andrew1.hunter/Vision/CV02ConnectedComponents.pdf.

Having determined the position and orientation of the connectedcomponent, the image processing module may perform one or more optionaldecision steps to facilitate determining the code type and value. In oneembodiment, optional decision steps 106, 110, 114, 118, and 120 are notused and the process flows sequentially from step 104 through codedetermining steps 108, 112, 116, 122, 124, to a step 126 that ranks theresults of the code determinations and selects the closest code typebased on statistical analysis. In a second embodiment, these optionaldecision steps are used to select the code determining steps that shouldbe executed. For example, an optional decision step 106 dictates whethera code determining step 108 should be executed. This embodiment thusfunctions as a filter to reduce the computations required for codedetection.

To ensure a code type is not inadvertently discarded, a combination ofthe above techniques may be used. The combined technique may employ theoptional decision steps to assign a “confidence” factor to each codetype that is determined. For example, the image processing system canassign a confidence factor based on statistical analysis of a connectedcomponent fitting within a specified radius, as determined at decisionstep 106. Similarly, the image processing module can assign anotherconfidence factor if the same connected component fits within aspecified square area as determined at decision step 110. Otherconfidence factors can also be determined for the equality of aconnected component minor axis to a specified length (as in decisionstep 114), the match of a connected component to a specified shape (asin decision step 118), or the proximity of bits to a connected component(as in decision step 120). In general, a confidence factor can bedetermined for each code type, and the confidence factor can be used indetermining the code type that best fits the current detected component.For example, a confidence factor can be determined from how well thepixels “read,” i.e., how clearly they cluster into black and whiteintensity levels, and also, how well the pixel values “read” as one of avalid bit code out of a codebook of valid codes. The confidence levelcan also be determined by whether the maximum and minimum pixelintensity levels differ by at least some predetermined value, since ifthe intensities differ by too small a value, it is possible that thepixels represent noise.

For exemplary purposes, FIG. 4 illustrates a combination of the firstand second embodiments being used to filter out those code types that donot meet a minimum criterion for each code type, while determining acode value for each code type in which the connected component meets atleast the minimum criterion. The optional decision steps may beperformed alone or in any combination to initially screen the connectedcomponent for possible matching code types. For those code types thatthe connected component possibly matches, the image processing modulepreferably determines variances of the connected component from eachpossible matching code type. The image processing module then ranks thedeterminations by best fit to select the most closely matching code typeand corresponding code value, as the code applied to the object.

As indicated above, in an optional decision step 106, the imageprocessing module tests the connected component for fit within aspecific radius or radial mask. If the lengths of the connectedcomponent axes and/or the sum of pixel values are within predefinedranges (i.e., the connected component fits within a radial mask), thenthe image processing module continues at a step 108 to determine theradial code. Details for determining the radial code are discussed belowwith regard to FIGS. 5 and 6. After determining the radial code, or ifthe connected component does not fit within the predefined radius orradial mask, processing continues at an optional decision step 110.

In optional decision step 110, the image processing module tests theconnected component for fit within a specified square area correspondingto die dimensions. If the connected component matches a rotated whitesquare template or if the primary and secondary axes are of equal lengthand correspond to predefined die dimensions (i.e., the connectedcomponent fits within the die dimensions), then the image processingmodule continues at a step 112 to determine the die spot pattern.Details for determining the die spot pattern are discussed below withregard to FIGS. 7 and 8. After determining the die spot pattern, or ifthe connected component does not fit within a specified square areacorresponding to the known die dimensions, processing continues at anoptional decision step 114.

In optional decision step 114, the image processing module tests theconnected component secondary axis for equality to a predeterminedlength corresponding to a linear code height. If the length of theconnected component short axis is equal to a predefined length (i.e.,the connected component secondary axis is equal to the linear codeheight), then the image processing module continues at a step 116 todetermine the linear code. Details for determining the linear code arediscussed below with regard to FIGS. 9 and 10. After determining thelinear code, or if the connected component secondary axis is not equalto a predetermined length, processing continues at an optional decisionstep 118.

In optional decision step 118, the image processing module tests theconnected component for a match with a predetermined shape or cuecomponent. If the connected component is not a linear shape with oneknown dimension (i.e., the connected component does not match the cuecomponent), then the image processing module continues at step 126,otherwise the image processing module continues at optional decisionstep 120.

In optional decision step 120, the image processing module tests theconnected component for nearby code bits. If the connected component hasdisconnected bits within a predetermined distance and orientation (i.e.,the connected component has nearby code bits), then the image processingmodule continues at step 122 to determine the matrix code. Details fordetermining the matrix code are discussed below with regard to FIGS. 11and 12. After determining the matrix code or if the connected componentdoes not have disconnected bits within a predetermined distance andorientation (i.e., the connected component does not have nearby codebits), then the image processing module continues at step 124 todetermine the gray scale code. Details for determining the gray scalecode are discussed below with regard to FIGS. 13, 14, and 15. (Note thatthe logic flow from step 122 to step 124 refers to the sequential logicflow of the first embodiment.)

In step 126, the image processing module ranks the results of the codedetermination(s), and then performs a statistical analysis to select thecode type that the connected component most likely fits. If theconnected component is not detected as a code, then the image processingmodule may evaluate the connected component as being another type ofobject that does not have an attached identifier, such as the finger ofthe user, or a non-coded object, etc.

Radial Code Recognition

FIG. 5 illustrates an exemplary radial code 130 that is particularlyuseful in providing an identifier for a round object, i.e., an objecthaving a round “foot print.” Radial code 130 comprises a lightreflective inner circular area 132 with a dark “start bit” 134. Startbit 134 is preferably located at a predefined first radius from thecenter of the coded region, and can take the shape of a keystone, a pieslice, or any other shape that makes the start bit easy to locate. Thestart bit within the light reflective inner circular area defines astarting reference point from which the code value can be determined.The start bit, in conjunction with the light reflective inner circulararea, also defines the orientation of the object to which the code isapplied.

Radial code 130 also comprises an outer, evenly divided concentricannulus 136 with light and dark keystone-shaped “data bits.” Annulus 136is disposed at a second radius from the center that is greater than thefirst radius. Note that the dashed lines are only included in thisFigure to more clearly indicate the disposition of the data bits.However, an actual radial code does not contain any such lines, and theentire light reflective area comprises a single connected component. Inthis case, the whole identifier code acts as its own cue component. Theouter, evenly divided concentric annulus contains the value of the codeas a series of light and dark keystone-shaped bits. The code value isread starting from the location of the start bit in a predefinedclockwise (or alternatively, in a predefined counterclockwise)direction.

A dark background 138 surrounds radial code 130 so that the radial codecan be easily detected as a connected component. The dark backgroundminimizes the risk of “noise” pixels being considered as part of theconnected component.

In FIG. 6, a flow diagram illustrates the logical steps performed by theimage processing module for processing a radial code. In a step 140, theimage processing module searches for the start bit at a predefinedradius from the center of the circular area. The search area is disposedwithin the radius of the data bits. In general, the image processingmodule can use a predefined fixed threshold to determine the value ofany bit. However to improve reliability, the image processing modulepreferably calculates a threshold as the mean between minimum andmaximum pixel values of the code pattern. As noted above, too small adifference between the minimum and maximum pixel values will likely becaused by an attempt to decode noise.

In a decision step 142, the image processing module determines whetherthe start bit was found at the predefined radius. If no start bit isfound, the process for radial code determination ends. However, if astart bit is found, the image processing module continues with a step144, reading the data bits at a predefined radius from the center of thecircular area in a predefined clockwise (or counterclockwise) direction.The bits are of known radial width relative to transformed tablecoordinates from the input image.

An optional decision step 146 tests for a valid radial code. To test thevalidity of a radial code, the image processing module can attempt toverify that the detected bits represent a code value that exists in atable of possible valid code values. Alternatively, the image processingmodule can compare the connected component to templates of valid radialcodes. This pattern matching technique is discussed in further detailwith regard to FIG. 8. If the code detected is not a valid one, theprocess for radial code determination ends. If the code detected isvalid, or if the optional validity check is not performed, the imageprocessing module continues to a step 148 and determines an object IDfrom the bits detected in the code using a table lookup or other suchmethod.

Die Matrix Code Recognition

FIG. 7 illustrates an exemplary matrix code 150 useful in recognizing asquare object, such as a die face. Matrix code 150 comprises a connectedcomponent square area 152 (which is shown as white) having from one tosix data bits 154 (die spots) arranged in six predetermined patternswithin a 3×3 grid 158. Note that the dashed lines are only included inthe figure to illustrate the data bits; an actual die matrix code wouldnot contain any lines.

Pixels of the die are read from the 3×3 grid within the square area. The3×3 grid pattern is then compared to each of the six allowable die facepatterns and, in the case of a “2,” “3,” or “6” spot die face, is alsocompared to a version of the die face pattern that is rotated 90degrees. Alternately, a grid of any size and associated die face bitpatterns can be read. Once the die face that is facing toward thedisplay surface is identified, the die face that is facing upward isknown, because opposite die faces always have a predefined relationship,i.e., the die face opposite a known die face has a number of spots equalto the difference between seven and the known die face.

A dark background 156 surrounds matrix code 150 so that the matrix codecan be easily detected as a connected component. The dark backgroundminimizes the risk of “noise” pixels from being considered as part ofthe connected component.

In FIG. 8, a flow diagram illustrates logic steps performed by an imageprocessing module for determining a die matrix code. In a step 160, theimage processing module determines the orientation of the squareconnected component by rotating a white square template at predeterminedangles through 90 degrees and matching the template to the connectedcomponent. The image processing module then continues at a step 162 toread the pixel pattern of the binarized image of the connected componentfrom within a 3×3 grid.

A step 164 a begins a process that iteratively examines each known dieface pattern (and a version of the die face pattern that is rotated 90degrees, in the cases of a die faces having “2,” “3,” or “6” spots). Ina decision step 166, the pixel pattern read from the connected componentis compared to a known die face pattern. If the pixel pattern matchesthe known die face pattern, the image processing module continues to astep 168 to determine the value of the top face of the die. As notedabove, the sum of opposite faces on a die equals seven, so the value ofthe top face equals seven minus the number of spots in the detectedpattern. After determining the top face value, the image processingmodule ends. If, however, no known die face pattern matches theconnected component pixel pattern, the iterative process terminates at astep 164 b.

Linear Code Recognition

FIG. 9 illustrates an exemplary linear code 170 useful in recognizingobjects. A linear code 172 comprises two light parallel bars 174 a and174 b, a light start bit 176, a dark stop bit 178, and a multiplicity oflight or dark code bits 180. It will be apparent that the terms “light”and “dark” refer to the IR reflectance of the material applied to createthe coded pattern on the object or on the material of the object itself.A light bit corresponds to a portion of the identifier that is a goodreflector of IR light, while a dark bit or area corresponds to a portionof the identifier or the surrounding portion of the object that absorbsor does not reflect IR light very well. These terms are somewhatarbitrary, since an image of the display surface can readily beelectronically inverted.

A dark background 182 surrounds the linear code so that the linear codecan be easily detected as a connected component. The dark backgroundminimizes the risk of “noise” pixels from being considered as part ofthe connected component. Again, note that the lines delineating the databits are only included in the figure to illustrate the data bits. Anactual linear code will typically not include any distinguishable lines.

Linear codes are connected components and are symmetric about a major orlong axis. The orientation as computed from the connected componentprocess may be 180 degrees off the true orientation of the object thatdisplays the code. By examining the location of the start and stop bits,the decoding algorithm can correct for the orientation. The number ofbits in a linear code is variable. Given the size (e.g., the width) of abit, the number of bits in a linear code can be determined directly fromthe length of the light parallel bars. Exemplary linear codes 172 athrough 172 h illustrate all eight instances of a 3-bit code.

In FIG. 10, a flow diagram illustrates the logical steps performed bythe image processing module for determining a linear code. In a step190, the image processing module locates the end edges of the connectedcomponent by starting at the center of the major (long) axis and summingpixel values in each line perpendicular to the major axis of theconnected component. An end of the linear component is ascertained whenthe sum is zero or less than or equal to a predetermined low threshold.The end with the start bit will have a sum greater than the end with theend bit. In a decision step 192, the image processing module tests forthe presence of the start bit. If a start bit is not found, the processfor determining a linear code terminates, otherwise the processcontinues with a step 194 to determine the length of the code from theends of the connected component found in step 190.

In an optional decision step 196, the image processing module tests fora valid code length. If the length of the code is not as expected, basedupon the software application in which the object is being detected, theprocess for determining a linear code terminates. If the length of thecode is as expected, as per the software application, or if the optionaldecision to test for a valid code length is not performed, the processcontinues with a step 198 to determine the number of bits in the code.The number of bits in the code is calculated from the formula:Code Bits=(Connected Component Length/Bit Width)−2

The process continues with a step 200, to determine the value of eachcode bit along the major or primary axis. A code bit value could bedetermined by any of the following:

-   -   based on the value of a pixel at the center of the code bit;    -   by comparing the mean value of the sum of all pixel values        within the area of the code bit with a predetermined threshold        value; if the mean value is greater than the predetermined        threshold value, then the code bit value is “1;”    -   by pattern matching; or    -   using some other technique.        The process continues at a step 202, to determine an object ID        from the value of the code bits via table lookup or other such        method.        Variable Bit-Length Matrix Code Recognition

FIG. 11 illustrates an exemplary variable bit-length matrix code 210useful in recognizing objects. A variable bit-length matrix code 212comprises a light rectangular-shaped connected “cue” component 214. Theminor or short axis of the cue component has a fixed length whereas themajor or long axis may be of any length (greater than the minor axis).The estimation of orientation of the variable bit-length matrix code isderived in much the same way as the linear code. The orientation of thecue component is correct up to a 180-degree rotation, which may then becorrected by determining the side of the cue component on which the bitfield is disposed. Preferably, the shape of the cue component improvesorientation reliability of the corresponding elliptical model.

The variable bit-length matrix code also comprises a matrix field 216,which is disposed a fixed distance away from the major axis of the cuecomponent on one or both side of the cue component. Note that the linesdelineating the data bits are only included in the figure to illustratethe data bits; an actual matrix field would not contain any visiblelines. The width of the matrix field is equal to the length of the majoraxis of the cue component. The matrix field includes one or more rows ofdata bits. Each row of the matrix field contains at least one light orwhite bit, so that a decoding algorithm may determine the number of rowsof bits by reading successive rows from the cue component outward untilno white bits are found in a row. Alternately, the number of rows in thematrix field may be encoded into the first row of the matrix field. Inconjunction with the variable length major axis of the cue component,the variable number of rows enables the code to be shaped appropriatelyto fit various object sizes. The matrix field can comprise one or morechecksum bits, such that after the bit sequence is decoded by imageanalysis, errors in the decoding may be detected. Checksum bits can alsobe used to determine whether the decoding process has read the bitsequence from the correct side of the cue component if there is no otherprocedure to determine that issue. Such an algorithm may simply attemptdecoding from both sides of the cue component, and use the validsequence as the correct bit pattern. Alternately, the shape of the cuecomponent can indicate the side of the matrix on which the code isdisposed.

The variable bit-length matrix code is surrounded by a dark background218. The dark background enables easy detection of the cue component asa connected component and minimizes the risk of “noise” pixels beingconsidered part of the cue component and matrix field.

In FIG. 12, a flow diagram illustrates logic steps performed by theimage processing module for determining a variable bit-length matrixcode. In a step 230, the image processing module determines the lengthof the cue component. The image processing module represents the cuecomponent as an ellipse and calculates a center and the lengths of themajor and minor axes of the ellipse. The image processing module thendetermines the position and orientation of the cue component relative totable coordinates, from the statistical mean and covariance of theelliptical axes.

The processing continues at a step 232 to determine the number of bitsper row of the matrix field. Because the width of code bits is fixed andknown, the number of bits per row is equal to the length of the cuecomponent divided by the known width of a code bit.

In a preferred optional step 234, the image processing module accessesthe normalized image instead of the binarized image to facilitatefurther processing. Binarizing the normalized image often introducesnoise pixels, and reading bits from the normalized image yields morereliable pixel values and enables smaller bit patterns to be read.

The processes within a block 236 a ascertain the value of the variablebit-length matrix code on one side of the cue component. A step 238 abegins a process that iteratively addresses each row of the matrix fieldon one side of the cue component. The iteration of reading the code bitsoccurs in discrete steps as each row of the matrix field is located atsuccessive predetermined offset distances from the cue component. In astep 240 a, the process reads the data bits in one row of the matrixfield. Reading a data bit is preferably done using a predefined fixedthreshold or a relative threshold, wherein the relative threshold isdetermined as a function of the differences between the maximum andminimum pixel intensities. Pattern matching is possible, but limits theflexibility of detecting variable length rows and a variable number ofrows.

A decision step 242 a tests for the lack of code bits, which indicatesthe end of the code. If the row contains no code bits, the iterativeprocess terminates, otherwise the process continues at a step 244 a, tostore the row code for later processing. The iterative process continuesat a step 246 a.

The optional processes within a block 236 b then ascertain the value ofthe variable bit-length matrix code on the opposite side of the cuecomponent. This step is executed if the application program expectsvariable bit-length matrix fields to be included on both side of the cuecomponent or if the application program attempts to read from both sidesof the cue component and uses an algorithm to validate the code using achecksum or other validation process, since the orientation of thematrix code is determined ±180 degrees.

An optional step 238 b begins a process that iteratively addresses eachrow of the matrix field on the opposite side of the cue component.Thereafter, optional steps 240 b, 242 b, 244 b, and 246 b perform thesame functions as steps 240 a, 242 a, 244 a, and 246 a, respectively,but for the bits on the other side of the cue component. The codedetermination process continues at a step 248, which determines theobject ID from the code bits stored in step 244 using a lookup table orother appropriate method.

Gray Scale Code and Arbitrary Shape Code Recognition

FIG. 13 illustrates exemplary gray scale codes 252 useful in recognizingobjects. Gray scale codes use multiple thresholds to detect black andwhite images as well as gray scale images. A gray scale code 252 acomprises a connected “cue” component 254 a. A dark background 256 asurrounds the gray scale code so that the gray scale code can be morereadily detected as a connected component. The dark background enableseasy detection of the cue component and minimizes the risk of “noise”pixels being considered as part of the cue component. Binarization ofthe input image sets the threshold to a predetermined level that rendersthe input image in black and white. Hence, the binarized cue componentis white and thus easily recognizable as a cue component (where graybits are white in the binarized image).

When a cue component is detected, a second predetermined threshold isapplied to the normalized image to reveal gray and white code bitswithin the cue component. This predetermined second threshold may alsobe a relative threshold, determined as explained above. Thus, binarizedcue components 254 a, 254 c, 254 e, and 254 g contain counterpart grayscale components 254 b, 254 d, 254 f, and 254 h, respectively. The grayand white data bits are disposed in predefined locations relative to thecue component. Note that the lines delineating the data bits are onlyincluded in the figure to illustrate the data bits. However, an actualgray scale code would typically not include any such visible lines.

Cue components for gray scale codes need not be rectangular, as shown byexemplary cue components 254 c and 254 g. Once a connected component isrecognized, the second predetermined threshold can reveal any gray scalecode bits within the connected component. Alternatively, gray scalecodes can use pattern recognition for locating cue components.

The embedded nature of gray scale codes enables using two connectedcomponents with a common center. Such an arrangement of connectedcomponents is highly unlikely to occur by chance and is readilydetected. Exemplary cue component 252 e includes a connected component255 e that is centered within another connected component 254 e. Theorientation of gray scale codes can be defined by stipulating thatcertain code bits within a cue component are white, gray, or uniquecombinations thereof. For example, the orientations of gray scalecomponents 254 b and 254 f are easily ascertained if all but one of thecorner bits is required to be gray. The shape of non-symmetrical cuecomponents, such as 254 c and 254 g, can clearly indicate theorientation of the gray scale code. Gray scale codes can containchecksum bits, such that after the bit sequence is decoded by imageanalysis, errors in the decoding may be detected.

FIG. 14 illustrates an exemplary arbitrary shape code 260 that is usefulin recognizing objects, especially oddly shaped objects. The arbitraryshape code 260 comprises a connected “cue” component 262, and one ormore data bits 264. The arbitrary shape code is surrounded by a darkbackground 268. The dark background enables easy detection of the cuecomponent and data bits and minimizes the risk of “noise” pixels beingconsidered as part of the code. The data bits of an arbitrary shape codeare placed in predefined locations relative to the cue component.Alternatively, data bits can be placed in predefined locations relativeto a previous data bit so that the location of each data bit ispredefined.

The arbitrary shape code can also comprise one or more void areas 266.Such void areas can be inherent in the shape of the physical object towhich the code is attached. A void area can influence the placement ofdata bits. For example, data bit 264 b is separated from data bits 264 aby void area 266. Alternatively, a void area can be associated with amechanism that can affect the code, such as in an object 300 thatincludes a button 302 that, when depressed against a force of a helicalspring 304, changes the value of one or more bits in the code, forexample, by moving an IR reflective area 306 into contact with thedisplay surface. Alternatively, the button can change the reflectance ofany regular bit on any of the aforementioned codes.

In FIG. 15, a flow diagram illustrates logic steps performed by theimage processing module for determining a gray scale or arbitrary shapecode after locating a connected component. In a step 270, the imageprocessing module accesses the current normalized image that containsshades of gray. In a step 272, the image processing module determinesthe minimum gray intensity within the connected component. This stepdifferentiates the dark background of a gray scale or arbitrary shapecode from the code within a gray scale or arbitrary shape code. A step274 indicates that the image processing module sets a second threshold Tdefined by the equation:T=(Maximum White Intensity−Minimum Gray Intensity)/2Applying this threshold to the normalized image distinguishes gray bitsfrom white bits. Steps 270, 272, and 274 are not required for readingbits of an arbitrary shape code. However, accessing the normalized imageprovides more accurate readings of small bits.

In a step 276, the image processing module accesses predefined patternsof expected bit locations. These predefined locations can apply to bitswithin the cue component and/or to predefined bit locations outside of,but relative to, the cue component. The image processing module readsthe data bits in the pattern that was predefined in step 276 relative tothe connected component shape and orientation, in a step 278. These databits are read using the second threshold.

In an optional decision step 280, the image processing module tests fora valid code value. If the code value is not valid, the processterminates, otherwise the process continues at a step 282 thatdetermines the object ID from the code bits via table lookup or othersuch method.

In another approach used in connection with the present invention, anencoded pattern on an object is recognized for arbitrary shapes. In thiscase, it is stipulated that all image pixels outside a region (butwithin some defined greater region) must be black, while image pixels onthe coded pattern are black or white (or gray). This process worksbetter in connection with the above-noted technique of validating thecode that is read against a known codebook to ensure that it is indeed avalid code.

Although the present invention has been described in connection with thepreferred form of practicing it, those of ordinary skill in the art willunderstand that many modifications can be made thereto within the scopeof the claims that follow. Accordingly, it is not intended that thescope of the invention in any way be limited by the above description,but instead be determined entirely by reference to the claims thatfollow.

1. A two-dimensional identifier applied to an object for encoding avalue so that the value is determinable when the two-dimensionalidentifier is placed adjacent to a surface sensing system, thetwo-dimensional identifier comprising: (a) a cue component comprising acontiguous radial area of a detectable material to which the surfacesensing system is responsive and which is approximated as an ellipsewhen detected by said surface sensing system, said ellipse having axesthat indicate an orientation of the two-dimensional identifier relativeto a coordinate system of the surface sensing system, the radial areaincluding a sub-area comprising the non-detectable material disposed ata first predefined radius from a center of the radial area, the sub-arearepresenting a start bit that indicates a start location from which thecode portion is to be read; (b) a code portion disposed in a predefinedlocation relative to the cue component, said code portion encoding thevalue with at least one binary element that is detectable by the surfacesensing system; and (c) a border region that encompasses the cuecomponent and the code portion, the border region comprising anon-detectable material that is not sensed as part of thetwo-dimensional identifier by the surface sensing system and whichfunctions as an interference mask around the cue component and the codeportion, to minimize noise.
 2. The two-dimensional identifier of claim1, wherein the detectable material comprises a reflective material thatreflects infrared light to which the surface sensing system responds. 3.The two-dimensional identifier of claim 1, wherein a radial code is usedfor encoding the value, and wherein the code portion is configured in aregion disposed at a second predefined radius from the center of theradial area, greater than the first predefined radius, so that the codeportion generally extends around the cue component.
 4. Thetwo-dimensional identifier of claim 1, wherein a variable length linearcode is used for encoding the value, and wherein: (a) the cue componentcomprises parallel strips of the detectable material that are ofsubstantially equal length and are connected at a first end by apredefined area of the detectable material used as a staff bit, theparallel strips being separated by the non-detectable material at asecond end opposite the first end, the non-detectable material at thesecond end being used as an end bit; and (b) the code portion comprisesa paftern of detectable areas and undetectable areas, each detectablearea and each undetectable area being of predefined dimensions anddisposed between the parallel strips and between the staff bit and theend bit.
 5. The two-dimensional identifier of claim 1, wherein avariable bit-length code is used for encoding the value, and wherein:(a) the cue component includes a predefined dimension and a variablelength dimension, said predefined dimension always being smaller thanthe variable length dimension, the variable length dimension indicatinga reference orientation and indicating an extent of the code portion;and (b) the code portion begins with a first binary element at apredefined offset from the cue component, and any additional binaryelements extend away from the first binary element in at least one of:(i) a direction generally parallel to the reference orientation of thecue component, to form a row of binary elements that ends at the extentof the code portion indicated by the variable length dimension; (ii) adirection generally perpendicular to the variable length dimension, toform a matrix of binary elements that end when a row does not include atleast one binary element that is detectable by the surface sensingsystem; and (iii) predefined directions and predefined distancesrelative to an immediately preceding binary element, to form a series ofbinary elements at predefined locations relative to each other.
 6. Thetwo-dimensional identifier of claim 1, wherein the value is encodedusing a multi-level, variable bit-length code, and wherein: (a) the cuecomponent is detectable at a binarization threshold, the cue componenthaving a predefined shape with at least one variable length dimension,and the at least one variable length dimension indicates a referenceorientation and indicates an extent of the code portion; and (b) thecode portion comprises at least one binary element that is detectable ata gray scale threshold, but is not detectable at the binarizationthreshold.
 7. The two-dimensional identifier of claim 6, wherein the atleast one binary element that is detectable at the gray scale thresholdis disposed within the cue component such that the cue component isuniform at the binarization threshold, but reveals the at least onebinary element at the gray scale threshold.
 8. The two-dimensionalidentifier of claim 6, wherein the code portion begins with a firstbinary element disposed at a predefined offset from a local origindisposed at a predefined location relative to the cue component, and anyadditional binary elements extend away from the first binary element inat least one of: (a) a direction generally parallel to an axis used bythe surface sensing system; and (b) predefined directions and predefineddistances relative to an immediately preceding binary element to form aseries of binary elements at predefined locations relative to each othersuch that the code portion comprises a predefined shape.
 9. A method fordetermining a value from a two-dimensional identifier applied to anobject when the object is placed adjacent to a surface of a surfacesensing system, comprising the steps of: (a) detecting a cue componentof the two-dimensional identifier, the cue component comprising acontiguous area that is detectable by the surface sensing system and isencompassed by a border region that is not sensed by the surface sensingsystem as being part of the two-dimensional identifier and whichfunctions as an interference mask around the cue component to minimizenoise; (b) approximating the cue component as an ellipse having a majoraxis and a minor axis, to determine a position and orientation of thecue component relative to the surface sensing system; (c) locating abeginning of a code portion of the two-dimensional identifier relativeto the position and orientation of the cue component, the value beingencoded in the code portion by a plurality of binary elements, each of apredefined area and the code portion also being encompassed by theborder region, which also functions as an interference mask around thecode portion to minimize noise; (d) detecting the plurality of binaryelements at predefined locations relative to one of the beginning of thecode portion and to each other, with the surface sensing system; (e)decoding the value that is encoded as a function of the plurality ofbinary elements that are detected; and (f) determining an objectidentifier associated with the value of the two-dimensional identifier.10. The method of claim 9, further comprising the steps of: (a)illuminating the two-dimensional identifier with infrared light; and (b)detecting infrared light reflected from the cue component and the codeportion of the two-dimensional identifier, to detect the orientation ofthe two-dimensional identifier and to decode the value encoded by theplurality of binary elements.
 11. The method of claim 10, furthercomprising the steps of: (a) producing a normalized image from thereflection of the infrared light to compensate for a non-uniformity ofthe infrared light; (b) producing a binarized image from the normalizedimage based on a predefined first light intensity threshold; (c) usingthe binarized image to determine the cue component; and (d) using thenormalized image to detect the plurality of binary elements based on asecond light intensity threshold.
 12. The method of claim 9, wherein thevalue is encoded using a radial code, further comprising the steps of:(a) determining that the cue component comprises a contiguous radialarea; (b) detecting that a sub-area of the contiguous radial area ismissing from the contiguous radial area at a first predefined radiusfrom a center of the radial area, the sub-area representing a start bitthat from which the beginning of the code portion is to be located; (c)locating the beginning of the code portion relative to the sub-area andat a second predefined radius from the center of the radial area, saidsecond predefined radius being greater than the first predefined radius;and (d) detecting the plurality of binary elements along an arc at thesecond predefined radius, said arc being generally concentric relativeto the cue component.
 13. The method of claim 9, wherein the identifiercorresponds to spots of a die, further comprising the steps of: (a)determining that the cue component comprises a square having dimensionsthat are substantially equal to predefined dimensions of the die; (b)determining the orientation of the cue component by rotating a squaretemplate until the square template generally aligns with the cuecomponent; and (c) detecting the plurality of binary elements atpredefined locations within the cue component, the predefined locationscorresponding to a three-by-three square grid of possible spots of thedie aligned with the orientation of the cue component.
 14. The method ofclaim 9, wherein the value is encoded using a variable length linearcode, further comprising the steps of: (a) determining that the cuecomponent comprises substantially parallel strips of equal length thatare connected at a first end by a predefined area that is used as astart bit, the strips being separated at a second end that is oppositethe first end by a second predefined area that is used as an end bit;(b) locating the beginning of the code portion relative to thepredefined area at the first end and between the strips; and (c)detecting the plurality of binary elements between the start bit and theend bit along a center axis that is generally centrally disposed betweenthe strips and is substantially parallel to the strips.
 15. The methodof claim 9, wherein the value is encoded using a variable bit-lengthcode, further comprising the steps of: (a) determining that the cuecomponent has a predefined dimension and a variable length dimension,said predefined dimension being smaller than the variable lengthdimension, the variable length dimension corresponding to a primary axisof the ellipse that approximates the cue component, which indicates areference orientation and an extent of the code portion; (b) determininga maximum possible number of binary elements in a row based on thevariable length dimension of the cue component; (c) locating thebeginning of the code portion disposed at a predefined offset from thecue component; and (d) detecting the plurality of binary elements in atleast one of: (i) a direction generally parallel to the referenceorientation of the code component, to form a row of binary elements thatends at the extent of the code portion indicated by the variable lengthdimension; (ii) a direction generally perpendicular to the variablelength dimension, to form multiple rows of binary elements that end whena row that does not include at least one binary element is detected; and(iii) predefined directions and predefined distances relative to animmediately preceding binary element, to form a series of binaryelements at predefined locations.
 16. The method of claim 9, wherein thevalue is encoded using a multi-level, variable bit-length code, furthercomprising the steps of: (a) detecting the cue component at abinarization threshold; and (b) detecting the code portion at a grayscale threshold that is not detectable at the binarization threshold.17. The method of claim 16, wherein the code portion that is detectableat the gray scale threshold is disposed within the cue component suchthat the cue component is uniform at the binarization threshold, butreveals the code portion at the gray scale threshold.
 18. The method ofclaim 16, wherein the cue component includes a predefined dimension anda variable length dimension, further comprising the steps of: (a)determining that the predefined dimension is smaller than the variablelength dimension, the variable length dimension corresponding to aprimary axis of the ellipse that approximates the cue component, whichindicates a reference orientation and an extent of the code portion; (b)determining a maximum possible number of binary elements in a row basedon the variable length dimension of the cue component; (c) locating abeginning of the code portion at a predefined offset from the cuecomponent; and (d) detecting the plurality of binary elements in atleast one of: (i) a direction generally parallel to the referenceorientation of the cue component, to form a row of binary elements thatends at an extent of the code portion indicated by the variable lengthdimension; (ii) a perpendicular direction to the variable lengthdimension to form a matrix of binary elements that end when a row doesnot include at least one binary element that is detectable; and (iii)predefined directions and predefined distances relative to animmediately preceding binary element to form a series of binary elementsat predefined locations.
 19. The method of claim 16, further comprisingthe steps of: (a) locating a beginning of the code portion at apredefined offset from a local origin, the local origin being disposedat a predefined location relative to the cue component; and (b)detecting the plurality of binary elements in at least one of: (i) adirection generally parallel to an axis of the surface sensing system;and (ii) predefined directions and predefined distances relative to animmediately preceding binary element to form a series of binary elementsat predefined locations relative to each other, such that the codeportion has a predefined shape.
 20. A memory medium having machinereadable instructions for carrying out the steps of claim
 9. 21. Asystem for determining a value from a two-dimensional identifier appliedto an object, comprising: (a) an interactive display surface having aninteractive side adjacent to which the object can be placed, and anopposite side; (b) a light source that directs infrared light toward theopposite side of the interactive display surface and through theinteractive display surface, to the interactive side; (c) a light sensordisposed to receive and sense infrared light reflected back from thepatterned object through the interactive display surface forming animage that includes the two-dimensional identifier applied to theobject; (d) a processor in communication with the light sensor; and (e)a memory in communication with the processor, the memory storing dataand machine instructions that cause the processor to carry out aplurality of functions, including: (i) detecting a cue component of thetwo-dimensional identifier with the light sensor, the cue componentcomprising a contiguous area that is detectable by the light sensor andis encompassed by a border region that is not sensed by the light sensoras being part of the two-dimensional identifier and which functions asan interference mask around the cue component to minimize noise; (ii)approximating the cue component as an ellipse having a major axis and aminor axis, to determine a position and orientation of the cue componentrelative to the interactive display surface; (iii) locating a beginningof a code portion of the two-dimensional identifier relative to theposition and orientation of the cue component, the value being encodedin the code portion by a plurality of binary elements, each of apredefined area and the code portion also being encompassed by theborder region, which also functions as an interference mask around thecode portion to minimize noise; (iv) detecting the plurality of binaryelements at predefined locations relative to one of the beginning of thecode portion and to each other, with the light sensor; and (v) decodingthe value that is encoded as a function of the plurality of binaryelements that are detected.
 22. The system of claim 21, wherein themachine language instructions further cause the processor to detectinfrared light reflected from the cue component and the code portion ofthe two-dimensional identifier with the light sensor, to detect theorientation of the two-dimensional identifier and to decode the valueencoded by the plurality of binary elements.
 23. The system of claim 22,wherein the machine language instructions further cause the processorto: (a) produce a normalized image from the reflection of the infraredlight to compensate for a non-uniformity of the infrared light; (b)produce a binarized image from the normalized image based on apredefined first light intensity threshold; (c) use the binarized imageto determine the cue component; and (d) use the normalized image todetect the plurality of binary elements based on a second lightintensity threshold.
 24. The system of claim 21, wherein the machinelanguage instructions further cause the processor to determine an objectidentifier associated with the value of the two-dimensional identifier.25. The system of claim 21, wherein the value is encoded using a radialcode, and wherein the machine language instructions further cause theprocessor to: (a) determine that the cue component comprises acontiguous radial area; (b) detect that a sub-area of the contiguousradial area is missing from the contiguous radial area at a firstpredefined radius from a center of the radial area, the sub-arearepresenting a start bit that from which the beginning of the codeportion is to be located; (c) locate the beginning of the code portionrelative to the sub-area and at a second predefined radius from thecenter of the radial area, said second predefined radius being greaterthan the first predefined radius; and (d) detect the plurality of binaryelements along an arc at the second predefined radius, said arc beinggenerally concentric relative to the cue component.
 26. The system ofclaim 21, wherein the identifier corresponds to spots of a die, andwherein the machine language instructions further cause the processorto: (a) determine that the cue component comprises a square havingdimensions that are substantially equal to predefined dimensions of thedie; (b) determine the orientation of the cue component by rotating asquare template until the square template generally aligns with the cuecomponent; and (c) detect the plurality of binary elements at predefinedlocations within the cue component, the predefined locationscorresponding to a three-by-three square grid of possible spots of thedie aligned with the orientation of the cue component.